1. Field of the Invention
The present invention relates to wireless communication systems that perform transmission timing offset control.
2. Description of Related Art
With regard to wireless communications, it has been proposed to perform adaptive transmission timing control in order to support reception timing synchronization among mobile stations at the base station (see Technical Report of the Institute of Electronics, Information and Communication Engineers, RCS 2003-141). In a multiple-access scheme according to DS-CDMA (Direct Sequence—Code Division Multiple Access), by ensuring reception timing synchronization among mobile stations at the base station, the orthogonality of base paths among mobile stations concurrently accessing the base station can be ensured, allowing frequency utilization efficiency to be improved.
The conventional adaptive transmission timing control is explained by making reference to FIGS. 9 to 11. FIG. 9 shows an example of a delay profile at the base station for mobile station MS#1, FIG. 10 shows examples of delay-profiles for mobile stations MS#1 and MS#2 as evaluated by the base station, with FIG. 11 showing the delay profiles after timing control.
In order to perform adaptive transmission timing control, first a signal pattern previously agreed on between transmitting and receiving sides (pilot signal) is inserted by the transmitting side. The receiving side takes the convolution sum of the received signal and the pilot signal to determine the correlation electric power (delay profile) over a specific period. This specific period is referred to as delay-profile calculation range W1 . . . (W1+S). Again, samples lying within this range that exceed a path level threshold are called arrival paths. In the example shown in FIG. 9, Path#1, Path#2, and Path#3 are observed as arrival paths. The path having the greatest electric power level among these arrival paths, Path#2, is treated as base path. (The method of treating the path with the smallest timing, Path#1, as base path can also be considered.)
The base station calculates the greatest propagation delay time (Tmax) of an arrival path and respective propagation delay times (Tmax-T1) and (Tmax-T2) of base paths T1 and T2 of mobile stations MS#1 and MS#2. It then appends (Tmax-T1) to control information destined for MS#1 and (Tmax-T2) to control information destined for MS#2, to communicate to each mobile station respective transmission timing offset values. Each mobile station demodulates the control information and transmits, offsetting its transmission timing by the transmission timing offset amount communicated by the base station. As shown in FIG. 11, the above actions serve to establish, at position Tmax, base path timing synchronization among the mobile stations at the base station.
FIGS. 12 and 13 show an example delay profile for a case where the transmission timing offset amount changes to the extent of falling outside the current delay-profile calculation range.
For the case of the transmission timing offset amount changing to such a great extent, a method as shown in FIG. 12 can be considered in which, simultaneously with the mobile station delaying transmission timing by just Tmax-T1, the base station, too, delays the delay-profile calculation range by Tmax-T1. However, if on the side of the mobile station demodulation of the control information containing the transmission timing offset information fails, preventing the side of the mobile station from updating the transmission timing offset, the base path position remains unshifted so that, as shown in FIG. 13, solely the delay-profile calculation range at the base station is shifted, causing the base path to fall outside the delay-profile calculation range. Consequently, the path is missed at the side of the base station, and the adaptive transmission timing control breaks down.
The goal of the present invention is to provide a wireless communication system that can perform adaptive transmission timing control without overlooking of paths on the side of the base station even in the case when the transmission timing offset amount changes to the extent of falling outside the current delay-profile calculation range, and the side of the mobile station fails to demodulate the control information containing the transmission timing offset amount.